Tar formation, Reduction and Technology of Tar during Biomass Gasification/ Pyrolysis – an Overview

DOI : 10.17577/IJERTV6IS080047

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Tar formation, Reduction and Technology of Tar during Biomass Gasification/ Pyrolysis – an Overview

Naveen Kumar Pattar*, Gowreesh S S#

PG Student*, M. Tech Thermal Engineering, Associate Professor#,

Department of Mechanical Engineering, JSSATE,Bangalore.,VTU Belgaum.

Abstract – Biomass is an important primary energy source as well as renewable energy source. As the most promising biomass utilization method, gasification/pyrolysis produces not only useful fuel gases, char and chemicals, but also some byproducts like fly ash, NOx, SO2 and tar. Tar in the product gases will condense at low temperature, and lead to thickened or blockage in fuel lines, filters and engines. Moreover, too much tar in product gases will reduce the utilization efficiency of biomass. Therefore, the reduction or decomposition of tar in biomass derived fuel gases is one of the biggest obstacles in its utilization for power generation. In this paper, we review the literatures pertaining tar formation, reduction and technology of tar during pyrolysis also tar reduction or destruction methods during biomass gasification/pyrolysis. On the basis of their characteristics, the current tar reduction or destruction methods can be broadly divided into five main groups: mechanism methods, self-modification, thermal cracking, catalyst cracking and plasma methods.


It is well known that biomass is one of the important primary and renewable energy sources. Moreover, biomass is neutral in carbon dioxide circulation, that is, the amount of carbon dioxide it consumed through photosynthesis is the same as that given off by combustion. With the depletion of fossil fuel sources as well as the global warming issues, the utilization of biomass has been more and more concerned. At present, biomass share in worlds total primary energy consumption is about 12%, as shown in Fig. 1. It is estimated that biomass share will be increased to near 15% by 2010 in developed countries.

The gasification/pyrolysis for producing syngas is regarded as one of the most promising options for utilizing biomass. The syngas from biomass can be not only directly used in gas turbine for power generation but also catalytically converted into methanol, dimethyl ether, FischerTropsch oils or other chemical products.

Research into the gasification of carbonaceous fuels has traditionally focused on the reactions between solid fuels (e.g. coal, char and biomass) and gaseous reactants (e.g. O2, CO2, H2O and H2). The follow-up reactions that occur after the initial gasification reaction take place between gaseous, or in some cases liquid, products. There is a significant amount of information available on these reactions, such as the water-gas-shift reaction (H2 + CO2

H2O + CO) and the methane reforming reaction (CH4 + H2O CO + 3 H2). Some other reactions, however, have attracted less attention in the past, as they are relatively unimportant in coal and char gasification: for example, the reaction (gasification or separation) of tar compounds, which is particularly important in biomass gasification. This has been an increasing area of research during the past decade because of the growing demand for a sustainable energy supply and more recently because of the desire to reduce greenhouse gas emissions.

Although plenty of studies have previously been carried out into decomposition reactions of biomass tars and the formation of tars in the hydro pyrolysis and hydrogasification of biomass, this information is not readily accessible.

In the gasification/pyrolysis process, with exception of generating useful products, many byproducts such as fly ash, NOx, SO2 and tar are also formed. Tar derived from biomass gasification or pyrolysis will be condensed as temperature is lower than its dew point, then block and foul process equipments like fuel lines, filters, engines and turbines. It was reported that tar content in the syngas from an air-blown circulating fluidized bed(CFB) biomass gasifier was about 10 g/m3. For other types of gasifier, tar content varied from about 0.5 to 100 g/m3. However, most applications of product gases require a low tar content, of the order 0.05 g/m3 or less. Hence, tar disposal becomes one of the most necessary and urgent problems during biomass gasification. Up to now, a great amount of

work concerning tar reduction or reforming has been reported. In this work, we reviewed a great amount of literature and broadly divided the tar removal technologies into five groups: mechanism methods like cyclone, filters (baffle, fabric, ceramic), granular beds, RPS, Electrostatic precipitators and Scrubbers; self-modification, selecting optimal operation parameters for gasifier or using a low tar gasifier; Catalytic cracking; Thermal cracking and Plasma methods (Pyro arc, Corona, Glidarc).

    1. Definition of tars and measuring techniques

      This paper uses a number of terms and definitions that require some explanation or need to be defined first. Even the word tars is far from unambiguous, as set out below.

      Definition of tars and measuring techniques

      For a discussion of the definition of tars and measuring techniques the reader is referred to an ECN report, No. C–04-014.In this report a conglomerate of Dutch research institutes and universities have developed a new classification system for tars and this is explained in detail. However, this standard is not universally used and therefore other definitions are also applicable.

      In the case of the present report it is important to note that comparing data on tar concentrations and conversion from different studies is hampered by the fact that they use different definitions of tars and different methods of analyzing them. Often the definition of tars used is a practical one, based on the method used to measure them, e.g.:

      condensable tars at temperature X tars that are soluble in solvent Y

      The present report sometimes compares data from different studies, e.g. on tar cracking under inert conditions, in CO2/H2O and in H2. The author estimates that the data from the various studies are broadly comparable, without considering the definitions and measuring techniques in detail: for more details on these the reader is referred to the original articles. When, in the authors estimation, the measuring technique or definition of tars rules out even a broad comparison of data from different studies, this will be mentioned.

      1. Primary, secondary and tertiary tars

        The terms primary, secondary, tertiary, primary reactions and secondary reactions are not used uniformly in the literature. The term secondary tar is particularly confusing, referring in some studies to the products of secondary reactions (i.e. all tars other than primary tars) and in others to an intermediate category of tars (between primary and tertiary tars).

        This study uses definitions that can be regarded as a compromise between the commonly used terms primary reactions and secondary reactions and the definitions of primary, secondary and tertiary tars used in the studies by Evans and Milne, as they relate their definitions to actual compounds. They do not give names for the follow-up reactions, however. Also, we have retained the term secondary reactions as it is in common use.

        The compositions of the three types of tar given in are:

        Primary products characterized by compounds derived from cellulose, hemicellulose or lignin, e.g. laevoglucose, hydroxyl acetaldehyde, furfurals and methoxy phenols

        Secondary products characterized by phenols and olefins

        Tertiary producs characterized by aromatic compounds without oxygen substituents, divided into: Alkalised tertiary products, including methyl derivatives of the aromatics, e.g. methylacenaphthylene, methylnaphthalene, toluene and indeneCondensed

        (simplified) tertiary products, i.e. the AHs/PAHs without substituents, e.g. benzene, naphthalene, acenaphthylene, anthracene, phenanthrene and pyrene

        The two reactions are not linked to these three types of tar:

        1. Primary reactions are those where biomass decomposes, generating tar compounds (primary tar). Pyrolysis oil or bio-oil is largely made up of primary tars, with some secondary tars

        2. Secondary reactions are the continued reactions of the primary tar and can take place

        both in the interior of the biomass particle and outside it, in the gas phase. Note that secondary reactions produce not only secondary but also tertiary tars

        1. Thermal cracking, heterogeneous thermal cracking and catalytic cracking

          In this study, cracking in the absence of a solid (i.e. in an empty, inert reactor) is referred to as thermal. In the presence of an inert solid it is referred to as heterogeneous thermal, and in the presence of an active solid as catalytic. Coal or char (such as activated carbon, anthracite, pyrolysis char, char in gasification ash) is regarded in this study as a catalyst (at high temperatures above 600°C) and also as a possible adsorbent (especially at low temperatures below 600°C). The 600°C dividing line is fairly arbitrary but probably lies in a region where the coal or char is not very active as a catalyst and does not adsorb tars to any significant extent.

        2. Decomposition reactions, thermal cracking, gasification, hydrogasification

          This report refers as consistently as possible to reactions of tars at high temperatures in general as tar decomposition reactions. The terms thermal cracking and gasification are used for decomposition reactions in an inert atmosphere and in the presence of O2 (in a limited amount), CO2, H2O and H2 respectively. Tar combustion reactions (with excess O2) are not considered in this report. For the specific reaction of tars with H2 the term hydrogasification is used. The difference between hydro pyrolysis and hydrogasification is explained in 3.5.1.

        3. Carbon/char/coke/dust/soot

        In this report the solid carbonaceous product of the thermal cracking of tars is referred to as carbon.This is only one of the many possible nomenclatures: others used i n the literature is char, coke, carbone (French), dust, soot and pyrocarbon.


        Mechanism methods include scrubber, filter, cyclone and electrostatic precipitator. The primary use of these devices is to capture particles from the product gases. A great amount of experimental results demonstrated that the methods were also considerably efficient in removing tar accompanied with effective particles capture. Tar separation efficiency ranging from 51% to 91% had been reported in a venturi scrubber used to purify the product gases from a countercurrent rice husk gasifier. Bridgewater

        claimed that tar concentration in the fuel gases was lower than 2040 mg/Nm3 after a high-efficient scrubber system.

        A so-called RPS (rotating particle separator) was used in Energy Research Center of Netherlands(ECN) with an attempt to remove tar from product gases, but the results was unsatisfactory. Another new tar removal system called OLGA (OLGA is the Dutch acronym for oil-based gas washer) was also developed by Boerrigter in ECN.The OLGA had been successfully demonstrated in a laboratory scale biomass gasifier. The results indicated that tar could

        Hasler summarized the tar and particle reduction efficiency by various mechanism methods in Table 1.

        As we know, the operating parameters play a very important role in the distribution of products during biomass gasification. The important parameters include temperature,

        Table 1

        The reduction efficiency of particle and tar in various gas cleaning system

        be selectively removed from the product gases without

        affecting the main gaseous products. In the OLGA, heavy tars were completely removed, which resulted in the dew

        Particle reduction (%)

        Tar reduction


        point decrease, even lower than 25 ºC. Therefore, tar would

        not condense at the downstream of gasifier. Furthermore, 99% phenol and 97% heterocyclic tars removal can be achieved, which was expected to be high enough to prevent excessive waste water treatment cost due to the pollution with phenol or other water-soluble tar compounds.

        Electrostatic precipitation(ESP) is one of the primary particle collection devices in coal fired power plant, metallurgical industry and cement industry due to its high efficiency. It is declared by Paasen that more than 99% dust and 4070% tar removal can be obtained by the ESP at an updraft gasifier in Harboore, a downdraft gasifier at Wiener Neustadt and a circulating fluidized bed gasifier at ECN. After passing ESP, the heavy tars in product gases were completely removed and the dew point of tar ranged from 130 to 21ºC, which was sufficiently low for preventing the condensation of tar. Based on visual inspection, the author claimed that fouling at the ESP collector plates was negligible. Moreover, it was found that tar removal was not sensitive to the voltage and residence time.

        Dinkelbach also experimentally studied wet ESP used in a Wellmann gasifier in Birmingham (UK). In the system, no operation problem had been found for a long term and the operators claimed to have obtained good tar separation efficiencies. Unfortunately, no detailed experimental data was available in the paper. The validity of ESP capturing tar was also confirmed by Neeft.

        Activated carbon is a highly efficient sorbent, and is widely used to control a number of gaseous pollutions emission. Hasler investigated the possibility of using activated carbon granular bed filter to remove tar. The activated carbon filter was installed in the front of a fabric filter. In the experiments, the removal efficiencies for high boiling hydrocarbons and phenols were relative high. Meanwhile, the tar laden activated carbon can be recycled as an extra feedstock. Hermann studied a pre-coated fabric filter used to remove particles and tar at a gasifier plant in Austria. The filter had been tested for more than 2500 h without any problems. The disadvantages of the filters were the following: the tar deposited in filter could not be easily cleaned; tar accumulation on the filter surface would lead to eventual plugging. Generally, barrier filters were not suitable for tar removal even though the filters were successfully demonstrated in some cases.


        Sand bed filter 7099 97


        Wash tower 6098 25


        Venturi scrubber 90

        Wet electrostatic 0

        precipitator 499 60


        Fabric filter 7095 50


        Rotational particle

        separator 8590 70

        Fixed bed tar adsorber 50

        equivalence ratio (ER), the type of biomass, pressure, gasifying medium and residence time etc. Certainly, the selection of parameters also depends on the type of gasifier.

        Researchers have conducted extensive studies concerning the influence of temperature on tar production during biomass gasification. Li reported that tar yield from biomass gasification decreased drastically from 15 to 0.54 g/Nm3 as the average temperature increased from 970 to 1090 K. NarvPaez also studied biomass gasification at different temperatures and found that the tar content at 700 and 800 1C were 19 and 5 g/Nm3, respectively. In the experiments of Fagbemi, tar yield was increased with the enhancement of temperature until to 600 1C, and then dropped with temperature increment. The phenomena can be explained the reasons: when the temperature was higher than 600 1C, the secondary reaction (i.e. tar cracking) prevailed, which leded to tar decomposition.

        Similar to temperature, Equivalence ratio(ER) increase also has a beneficial effect on reducing tar formation. However, the heat value of product gases will decrease with enhancing ER. Lv divided the biomass gasification into two stages based on the ER. In the first stage, ER varied from 0.19 to 0.23. When ER shifted from 0.19 to 0.23, gas yield also was increased from 2.13 to 2.37 Nm3/(kg biomass) and gas low heat value(LHV) was increased from 8817 to 8839 kJ/Nm3. The ER range of the

        second stage was 0.230.27. In the stage, gas LHV decreased with ER increment because of strengthening oxidization reactions of product gases. Garca-Ibanez reported that the maximum amount of H2 (9.3 vol%) occurred at an ER of 0.59 and ER had slight effect on the hydrocarbons content at the ER range of 0.590.73.

        Kosstrin investigated the relation of maximum tar yield with the type of biomass through experiments. It was obtained that the highest yield of tar was 35% for wood, around 60% for paper and only 30% for sawdust. A similar research was also conducted by Sadakata, who studied the conversion of wood, lignin and holocellulose at a heating rate exceeding 1000ºC/min. The maximum tar yield obtained by holocellulose was higher than that of other materials.

        Devinder described the effect of steam content on tar

        formation during biomass gasification through a thermodynamic model. The simulation predicted that the more steam, the higher is the conversion efficiency of tar. Turn, Zainal and Lv also declared that the CO and H2 fraction in product gases increased with steam/carbon ratio enhancement

        Knight carried out biomass gasification under different pressures. Phenol was completely eliminated when the pressure was above 21.4 bar. However, the fraction of PAH increased with enhancing pressure though total tar decreased.

        Besides affecting the fraction of tar during biomass gasification, operation parameters also influenced the tar properties. Paasen revealed that tar concentration decreased with temperature varying from 750 to 950 ºC. Simultaneously, tar compositions shifted from alkyl- substituted poly-aromatic hydrocarbons(PAHs) to non- substituted PAHs.

        Yu pyrolysed birch wood in a free-fall reactor to observe the temperature effect on the gasification process. He found that the amount of substituted 1- and 2-ring aromatics drastically went down with increasing temperature, and 3- and 4-ring aromatics yield was increased accordingly. Brage [ reported an almost complete reduction of phenol content, 50% decrease in toluene content can be obtained when the temperature was raised from 700 to 900ºC. However, benzene and naphthalene had inverse tendency, their contents varied from 14 to 24 mg/l and 2 to 8 mg/l, respectively.

        Sousa performed tests to study how ER affected tar formation during wood gasification. The experimental results showed that the tar was a complex mixture of polycyclic aromatic compounds (benzene making 25% of the tars by mass, naphthalene 5.1%, indene 4.9%, acenaphthylene 1.7% and phenanthrene 1.4%), alkylated aromatic compounds (toluene 13.6%, styrene 5.3% and xylenes 5.2%) and phenolic compounds (phenol 15.1%, cresols 11.1%) at ER ¼ 0.2. When ER was increased to 0.3, the composition became less complex. The primary component was polycyclic aromatic compounds (benzene 42.9%, naphthalene 14.7%, indene 5.2%, acenaphthylene 4.4% and phenanthrene 3.9%), and the concentrations of alkylated aromatic compounds decreased remarkably. As

        for phenolic compounds, cresols completely disappeared and only a small amount of phenol was detected. At ER ¼ 0.4, the tar was decomposed almost exclusively of benzene(60% of the tars by mass), naphthalene (17%) and a small amount of three and four ringed polycyclic aromatic hydrocarbons. Contrary to Sousa, Houben reported that increasing ER led to the formation of higher ring aromatic components.


          In thermal cracking method, the raw gases derived from gasification or pyrolysis were heated to a high temperature, where tar molecules can be cracked into lighter gases. Bridegwater viewed that tar could be reduced by thermal cracking in a fluidized bed gasifier. Meanwhile, the author also mentioned that biomass-derived tar was very refractory and hard to crack by thermal treatment alone. In order to effectively decompose the tar, the following ways were suggested: increasing residence time, such as using a fluidized bed reactor freeboard, but this method was only partially effective; Direct contacting with an independently heated hot surface, which required a significant energy supply and decreased the overall efficiency. At the same time, the method was also partly effective and depended on good mixing; Partial oxidation by adding air or oxygen could increase CO levels at the expense of conversion efficiency decrease and operation cost enhancement.

          To achieve a sufficiently high tar cracking efficiency, Brandt claimed that the necessary temperature and residence time were 1250 ºC and 0.5 s, respectively. Tar and soot content at 1200, 1250 and 1290 ºC ,According to the review of Beenackers and Manuatis, the preferable tar content in gases for engine application was below 50 mg/Nm3. Hence, 1250 ºC was the limited temperature for tar decomposition.

          Houben also carried out thermal tar cracking experiment at temperature range of 9001150 ºC and residence time between 1 and 12 s. In the experiment, naphthalene carried by nitrogen was used as model tar. The maximal tar reduction reached 9899% at 900 ºC with an excess air ratio of 0.5.

            1. Catalyst cracking

              Due to the advantages of converting tar into useful gases and adjusting the compositions of product gases, catalyst cracking has been of interest since the middle 1980s. The simplified mechanism for catalyst tar reforming can be described as follows. First, methane or other hydrocarbons are dissociatively adsorbed onto a metal site where metal- catalyzed dehydrogenation occurs. Water is also dissociatively adsorbed onto the ceramic support, hydroxylating the surface. At the appropriate temperature, the OH radicals migrate to the metal sites, leading to oxidation of the intermediate hydrocarbon fragments and surface carbon to CO+H2. David summarized the criteria for catalyst as follows:

              1. the catalysts must be effective in removing tar;

              2. if the desired product was syngas, the catalysts must be capable of reforming methane;

              G The catalysts should provide a suitable syngas ratio for the intended process;

              H the catalysts should be resistant to deactivation as a result of carbon fouling and sintering;

              I the catalysts should be easily regenerated. J The catalysts should be strong; and

              K the catalysts should be inexpensive.

              Moreover, David reviewed tar catalyst cracking and divided the catalysts into three groups: dolomite catalysts; alkali metal and other metal catalysts; nickel catalysts. After several years of developing, some new catalysts have been applied in gasification. Here, we group the catalysts into four groups and detailed description can be found in the following.

            2. Ni-based catalyst

              Ni-based catalysts are extensively applied in the petrochemical industry for naphtha and methane reforming. Meanwhile, a wide variety of Ni-based catalysts are commercially available. Especially, some studies showed that nickel based catalysts had the ability of reversing ammonia reaction, thus it is possible to reduce NOx emission during biomass gasification.

              Zhang investigated tar catalytic destruction in a tar conversion system consisting of a guard bed and catalytic reactor. Three Ni-ased catalysts (ICI46-1, Z409 and RZ409) were proven to be effective in eliminating heavy tars (499% destruction efficiency). Hydrogen yield was also improved by 611 vol%(dry basis). The experimental results also demonstrated that space velocity had little effect on gas compositions, while increasing temperature boosted hydrogen ield and reduced light hydrocarbons (CH4 and C2H4) formation, which suggested that tar decomposition was controlled by chemical kinetics.

              Coll also studied the model compounds like benzene, toluene, naphthalene, anthracene, and pyrene were cracked using two commercial nickel catalysts: UCG90-C and ICI46-1 at 700800 ºC. The order of these model tars reactivity was: benzene4to- luene4anthracene4pyrene4naphthalene.Toluene conversion rate ranged from 40% to 80% with the ICI46-1 catalyst, and 20% to 60% for the UCI G90-C catalyst.

              Simell and co-workers reported the use of alumina and other catalysts with variable Ni content reformed toluene in various gas atmospheres at 900 ºC and0.520 MPa. The effects of sulfur poisoning on the activity of these catalysts for tar and ammonia decomposition had also been evaluated.

              Nickel supported on silica was active for tar catalyst cracking at relatively low temperature (823 K) was described by Zhang. However, these catalysts only maintained their activities for a short time because of accumulating large amounts of carbon on their surfaces. Aznar and Baker also mentioned the phenomena in their experiments. In order to overcome the shortcoming of the commercial Ni-based catalyst, many Ni-based catalysts were developed.

              Dou compared five catalysts on tar removal from fuel gases in a fixed-bed reactor. The Y-zeolite and Ni-Mo catalysts were found to be the most effective, such that 100% tar removal can be achieved at 550 ºC. It was also observed that process variables like temperature and space velocity had very significant effect on tar removal. The visual observation demonstrated that only very small amount of coke appeared at the surface of catalyst even with 168 h operation. The result of Marino indicated that the addition of Ni into Cu/Ni/KgAl2O3 catalyst was favorable to gases yield increase and acetic acid production reduction during ethanol gasification.

              Magnesium, lanthanum, and titanium oxide-doped nickelchromium/alumina catalysts were prepared by Denis, and experiments were performed to assess the performance of these catalysts in steam reforming naphthalene. The experimental results revealed that the improved catalyst could promote conversion efficiency of naphthalene. After the structure analysis, it was found that MgO had a significant effect on the robustness of catalyst due to the formation of MgAl2O4 spinel phase.

              Courson also developed a new Nibased catalyst by impregnating nickel oxide on olivine and calcination at 900, 1100 and 1400 ºC. X-ray diffraction, scanning electron microscopy and transmission electron microscopy coupled to energy dispersive X-ray spectroscopy analysis showed that there were interactions between the precursor and the support, which was consistent with the conclusion of Denis. After the characteristic studies, the catalyst performance tests indicated that the catalyst containing 2.8 wt% Ni calcined at 1100 ºC was the optimum catalyst. Furthermore, no sintering and very little carbon deposition were observed on this catalytic surface.

              Chen investigated CO2 reforming methane over NiO/g Al2O3 catalyst in a fixed/ fluidized bed. Francisco also compared the Ni catalyst supported on a-Al2O3, ZrO2 and a-Al2O3ZrO2, and found Ni/aAl2O3ZrO2 catalyst showed better performance. In the literature of Karen, he mentioned that the 1 wt%/0.5 wt% nickel/calcium catalyst co-precipitated inside porous filter discs can effectively remove tar (498%) even in the presence of 100 ppm H2S.

            3. Alkali metal catalysts

              Besides Nibased catalysts, many literatures proved that alkali metal catalysts were also effective in reforming tar. McKee successfully demonstrated that carbonates, oxides and hydroxides of alkali metals can effectively decompose tar during catalytic gasification.

              Gong also studied waste paper gasification in carbon dioxide atmosphere with molten alkali metal carbonates including potassium, sodium, lithium carbonate or their intermixtures as catalyst. The molten catalysts were capable of facilitating a desired reaction(C+CO2-2CO), which was hardly feasible even at a high temperature of 973 K without catalysts. Further experimental results demonstrated that the intermixed carbonates exhibited stronger enhancement on catalytic ability than any carbonate salts in pure form. Waste paper gasification catalyzed by molten alkali carbonates was also investigated by Roman.

              In the experiment of Demirba, three different biomass samples (cotton cocoon shell, tea factory waste and olive husk) were decomposed by direct and catalytic pyrolysis process to obtain hydrogen rich gaseous products at 775, 925, 975 and 1025 K. In the catalytic pyrolysis process, the yield of hydrogen rich gases was increased in the case of using ZnCl2 catalyst though the yield of charcoal and liquid products were also increased. While the effect of K2CO3 and Na2CO3 on pyrolysis products depended on the biomass species. It was found that the Na2CO3 was better than K2CO3 for the cotton cocoon shell and tea factory waste pyrolysis. Nevertheless, in the case of olive husk pyrolysis, the conclusion is opposite, and K2CO3 was more effective than Na2CO3. At the same time, the effect of the amount of Na2CO3 and K2CO3 on the pyrolysis products was irregular.

              Pant and Kumar also pyrolysed n-heptane over different calcium aluminates and potassium impregnated calcium aluminate in a fixed bed reactor. Compared to the unpromoted catalysts, K2CO3 impregnated catalyst significantly suppressed the coke deposited on the catalyst surface, but only had marginal effect on the product selectiveness. Brown also found alkali metal salts, especially those containing potassium, were excellent promoters for gasification reactions. On the contrary to the conclusion of Brown, a gravimetric study of the alkali metal hydridotetracarbonyl-ferrates catalytic activity at 800

              ºC revealed the following sequence with respect to the cation activity: Na4Li4 K4Rb4Cs.

            4. Dolomite catalysts

              Dolomite is a calcium magnesium ore with general chemical formula CaMg (CO3)2, and is generally used as raw material in the manufacture of magnesium. In recent years, it has been discovered that calcined dolomite is also a highly efficient catalyst for removing tar from the product gases of gasifier.

              Simell compared a commercially available metal based catalyst(NiMo/gAl2O3) with non-metallic mineral catalysts during the catalytic pyrolysis of toluene. The non- metallic mineral catalysts included Norwegian dolomitic magnesium oxide [MgO], Swedish low surface quicklime[CaO], and calcined dolomite [CaMg(O)2]. Among these catalysts, the catalytic effect followed the sequence: CaO4CaMg(O)24MgO4NiMo/gAl2O3.

              Rui and Rapagn claimed that the presence of dolomite in the fluidized bed had the benefit of decreasing tar content and rising gases yield. However, dolomite could not affect gaseous hydrocarbons concentrations. It was reported that an amount of 2030 wt% dolomite (rest being silica sand) in the gasifier reduced tar content to about 1 g/m3 at an ER of 0.3. The authors also studied the influence of several operating parameters combined with using in-bed dolomite.

              Devi reported that untreated olivine could convert only 46% tar in the hot gasification gases, which could not be considered as a significant reduction. While catalyst was pre-treated olivine, the conversion of naphthalene, considered as a model biomass tar compound, was as high as 80%. The pretreatment was only heating the olivine catalyst at 900 1C in the presence of air. It was expected that the calcination could activate olivine.

              Karlsson successfully demonstrated biomass integrated gasification with combined cycle (IGCC) process with dolomite as bed material. Only about 12 g/m3 of light tars (excluding benzene) and 100300 mg/m3 of heavy tars were detected in the product gases.

              Srinakruang and Wang developed a new catalyst (Ni supported by dolomite), which could maintain high activity and stability for a long contact time. Moreover, carbo deposition at the Ni/dolomite catalysts surface was negligible. The authors also claimed that the calcination temperature significantly influenced the property and activity of the Ni/dolomite catalyst since nickel oxide had strong interaction with the dolomite surface.

              Although the dolomite can effectively remove tar in some cases, there are still many problems during biomass gasification. Zhang reviewed the shortcomings of dolomite as the following: The conversion rate of tar catalyzed by dolomite was difficult to reach or exceed 9095%; Although dolomite could reduce the tar in syngas and change the distribution of tar compositions, it was difficult to convert the heavy tars by dolomite; The dolomite would be inactive since the particle was easily broken during gasification; The melting point of dolomite was low and the catalyst would be inactive resulting from the melting of dolomite.

            5. Novel metal catalysts

          Ni-based catalysts and dolomite were deactivated significantly by carbon deposition and alkali metal was easily sintered. Novel metals had been wildly used as catalyst for NOx and SO2 since 1980s. Some researchers found that the novel metal catalysts were able to overcome the shortcomings of conventional catalyst, and keep high efficiency on converting tar. Tomishige compared the tar conversion rates over M/CeO2/SiO2 (M ¼ Rh, Pd, Pt, Ru, Ni) catalyst during cellulose gasification. The order of catalyst activity in the cedar wood gasification at 823 K was the following: Rh4Pd4Pt4Ni ¼ Ru. The tar conversion rate was about 88% in the case of Rh/CeO2/ SiO2 catalyst at 823 K, which jumped to the 97% at 873 K. Since the amount of char on Rh/CeO2/SiO2 catalyst surface was very small at low temperature, no deactivation was observed during the operation period. In addition, Rh/CeO2/SiO2 exhibited high and stable activity even under the presence of high concentration of H2S (280 ppm).

          Asadullah and coworker also studied the performance of various kinds of Rh/CeO2/M-type(M ¼ SiO2, Al2O3, and ZrO2) catalysts for cellulose gasification in a continuous- feeding fluidized-bed reactor. Among the catalysts, Rh/CeO2/SiO2 exhibited the best performance with respect to generating syngas or hydrogen. Moreover, Pt, Ru, Pd, and Ni doped on CeO2, and supported by SiO2, Al2O3, TiO2, MgO, and ZrO2 were also tested and the results also proved that Rh/CeO2 was the best catalyst. However, in the continuous-feeding system, it was found that the Rh/CeO2 catalyst suddenly deactivated due to a decrease in surface area from 60 to 13 m2/g. After further study, the authors found that the loading of CeO2 on the high-surface-area SiO2 could inhibit the aggregation of CeO2 and maintain the catalytic activity. Among various loadings, 35%wt CeO2 on SiO2 was the most suitable support for Rh in terms of the tar conversion, gas yield, and fast char conversion.

          The catalytic performances of Co catalysts for the steam reforming of naphthalene were reported by Furusawa. The characterizations analysis (TPR, XRD, CO adsorp-tion, and CO-TPD) of catalysts showed that the large-sized Co metal particles were formed over the precalcined catalysts.

          Hao investigated Ru/C, Pd/C, CeO2 particles, nano-CeO2 and nano-(CeZr)xO2 catalytic cracking tar during cellulose and sawdust gasification. The experimental results demonstrated that the catalyst activities followed the order: Ru/C4Pd/C4nano-(CeZr)xO24nano-CeO2 4CeO2. Rh supported on CeO2, ZrO2 and SiO2 single metal oxides and various mixed metal oxides such as CeO2/SiO2, ZrO2/SiO2 and CeO2/ZrO2 were compared by Polychronopoulos, and found that 1.5 wt% Rh/CeO2/ZrO2 catalyst, the support of which was prepared by the solgel method, exhibited better performance than other catalysts.

          Sutton also studied the activity of the 3:17 Ni/Al co- precipitated catalyst with 1 wt% Ru/Al2O3 and 1 wt% Pt/ZrO2 for dry reforming CH4 and C3H8 at 450800 1C. Rapagna developed a catalyst with a chemical formula of LaNi0.3Fe0.7O3, which was prepared by means of a solgel related process, where La, Ni, and Fe nitrate salts were dissolved separately in hot propionic acid. The catalyst displayed high CH4 reforming activity at 800ºC. Garcia also reported that cobalt-promoted and chromium- promoted nickel catalysts supported on a MgOLa O a-Al O performed the best in terms of H2 yield and lifetime.

        2. CONCLUSIONS

This study has collected the information on the reactivity, tar formation and the technology of the tar during pyrolysis/Gasification process from the published journals.

Although the primary use of mechanism methods is to capture the fly ash or particles from the product gases, the effect of gas removal is very good about 40-99% of tar can be reduced by different mechanism methods.

The Gasification/Pyrolysis not only produces only useful fuel gases, but also same products like fly ash, NOx, SO2 and tar. The different type of cracking methods such as thermal cracking where in the raw gases derived from the gasification where heated to high temperature where the tar molecules cracked into the lighter gases, catalyst cracking, Ni based catalyst, Alkali metal catalyst, Dolomite catalyst and Novel metal catalyst method of cracking were studied in the above work.


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